1. Field of the Invention
The present invention relates to an electrophotographic photosensitive element, and more specifically, it relates to an electrophotographic photosensitive element equipped with a photosensitive layer containing a charge generating materials and a charge transport material.
2. Description of the Prior Art
As photoconductive materials of photosensitive elements which have been heretofore used in an electrophotographic system, there are inorganic materials such as selenium (Se), cadmium sulfide (CdS), zinc oxide (ZnO) and amorphous silicon (a-Si). These inorganic photosensitive elements have many advantages, but simultaneously they have various disadvantages such as being toxic, the difficulty of disposal and high cost. Therefore, in recent years, many organic photosensitive elements using organic materials which are free from these disadvantages have been suggested and already put into practice.
These photosensitive elements can be classified into a function separating type photosensitive element having a multi-layer structure in which a material for generating a charge carrier (hereinafter referred to as "charge generating material") and another material for receiving the generated charge carrier and transport it (hereinafter referred to as "charge transport material") are used in separate layers, and a single layer type photosensitive element in which the charge generation and the charge transport are carried out in one layer. In general, the function separating type photosensitive element is more often used because of having a wide selectable range of materials and being capable of heightening sensitivity.
As charge transport materials, there are a material using a photoconductive polymer such as a polyvinylcarbazole, and a material obtained by dispersing/dissolving a low-molecular photosensitive material in a binder polymer. The photoconductive polymer, when used singly, is insufficient in film forming ability and adhesive properties, and in order to improve these points, a plasticizer, a binder polymer and the like are used. However, the employment of these components may give rise to the deterioration of the sensitivity and the increase of residual surface potential, which are practical problems. On the other hand, in the case that the low-molecular photosensitive material is dispersed/dissolved in the binder polymer, the photosensitive element having excellent mechanical properties can easily be obtained by suitably selecting the polymer. However, the thus obtained photosensitive element is not considered to be sufficient in sensitivity and stability. For example, a diaryl alkane derivative described in U.S. Pat. No. 3,820,989 has little problem of compatibility with the binder polymer, but it is poor in stability to light. Thus, if this derivative is used for the photosensitive layer of the electrophotographic photosensitive element and then repeatedly subjected to charging and exposure, the sensitivity of the photosensitive element gradually deteriorates.
Furthermore, in the case that stilbene compounds described in Japanese Laid-open Application No. 65440/1983 are repeatedly used, stability is not satisfactory, though charge retention ability, sensitivity and the like are relatively good. In addition, these compounds have a problem regarding the sensitivity at low surface potential.